This invention relates to improved rotors for internal batch mixing machines having two counter-rotating, non-intermeshing four wing rotors. The four-winged rotors of this invention provide improved dispersive and distributive mixing of materials in the batch mixer. The invention also relates to batch mixing machines employing two of the new four-wing rotors of this invention, and to improved batch mixing utilizing such batch mixing machines having the improved four-wing rotors.
This invention relates to high intensity internal mixing machines of the batch type having a mixing chamber shaped to accommodate two counter-rotating non-intermeshing winged rotors. The batch of ingredients to be mixed into a homogeneous mass is fed down into the mixing chamber through a vertical chute and is pushed down under pressure by a ram located in the chute. This ram is hydraulically or pneumatically driven. The lower face of the ram, when advanced down to its operating position during mixing of the batch, forms an upper portion of the mixing chamber. The homogeneous mixture produced is removed from the mixing chamber through a discharge opening at the bottom of the chamber, and a door associated with this opening is then closed in readiness for the next batch of ingredients to be introduced down through the chute.
Some internal batch mixing machines are designed with non-intermeshing rotors, and others have intermeshing rotors. Intermeshing rotors must always be driven at the same rotational speed in synchronized relationship; non-intermeshing rotors may be driven at the same rotational speed or at different rotational speeds for achieving different mixing and kneading effects. The present invention relates to the non-intermeshing type. The wings of the rotors have a generally helical configuration, and they produce high intensity mixing and homogenization by the co-operative interaction of their various forceful dynamic effects, as described later. For further information about such internal batch mixers, having non-intermeshing rotors, reference may be made to U.S. Pat. Nos. 1,200,070 and 3,610,585, assigned to predecessors of the present assignee and to recent U.S. Pat. Nos. 4,744,668 and 4,834,543, and the disclosures of these patents are incorporated herein by reference as background information.
A large majority of all internal batch mixing machines in commercial usage today in the United States having non-intermeshing rotors are operated non-synchronously, i.e. with the rotors being driven at different rotational speeds, often called xe2x80x9cfriction ratioxe2x80x9d operating mode. For example, a typical non-synchronous operation causes one rotor to make 9 revolutions versus 8 revolutions for the other rotor, i.e. a xe2x80x9cfriction ratioxe2x80x9d of 9 to 8 or 1.125 to 1.
In U.S. Pat. No. 4,744,668, issued May 17, 1988, are described novel four-wing and three-wing rotors of increased performance adapted for use in either the currently more numerous non-synchronous batch mixers or synchronous batch mixers.
U.S. Pat. No. 4,834,543 describes four-wing, non-intermeshing rotors to be driven at synchronous speed at a constant 180xc2x0 phase angle, with each of the two rotors used in the batch mixing machine having two long wings and two short wings on each of the two rotors.
In both U.S. Pat. Nos. 4,744,668 and 4,834,543, there is a recognition that optimum or preferred results are achieved by driving the specified rotors synchronously while oriented in a preferred phase angle relationship. The patent specifies that preferred phase angle relationship as being about 180xc2x0.
The four-wing, non-intermeshing (tangential) rotors heretofore proposed for use with synchronous rotation internal batch mixing machines have either been of the type that promote primarily micro dispersive (intensive) mixing due to the intensive shear forces generated by the rotors in the mixer chamber. In dispersive mixing the high shear forces generated rapidly break down agglomerates in the batch of materials to be mixed. The non-intermeshing rotors also have the properties of providing high fill factors and short feeding and discharge times along with the excellent dispersive mixing characteristics. However, such non-intermeshing rotors do not provide essentially equally good distributive (extensive) mixing of the materials to be mixed. Also, use of such non-intermeshing rotors can be characterized by an undesirable temperature rise in the material to be mixed.
On the other hand, mixing machines heretofore employing intermeshing rotors have better heat transfer characteristics and better thermal control over the mixing batch. Also, in contrast to the mixing machines employing the non-intermeshing rotors, the machines employing intermeshing rotors exert high elongational deformations in the nip region between the two rotors producing good stream splitting and thus good distributive mixing. In contrast, in the mixing machine employing non-intermeshing rotors only mild stream splitting is produced in this region and therefore generally does not produce essentially equivalent distributive mixing. There is therefore a need for rotors for use in batch mixers that simultaneously produce both good dispersive and good distributive mixing in the processing of the batch of materials to be mixed and thereby obtain the benefit of both intermeshing and non-intermeshing rotors.
This invention provides a new four-wing rotor design in which each rotor wing performs a specific function, and use of these rotors as synchronously driven rotors in mixing machines to produce both good dispersive and good distributive mixing of the mixing batch and good process temperature control, and thereby provide better utilization of the mixing chamber of the mixer and yield a more thermally and compositionally homogeneous mixed product. In the rotors of this invention certain wings promote primarily dispersive mixing and certain wings promote primarily distributive mixing in the batch.
A further feature of this invention is that use of the new rotors in the mixing machines enforces certain flow patterns in the window of interaction between the two rotors in the mixer and produces more efficient exchange of material between one rotor chamber and the other rotor chamber of the mixer. This function is accomplished in part by a rotor with wings having a substantial helical length such that there is present in the window of interaction between the two rotors a rotor wing almost at all times. This allows great flexibility in affecting the flow patterns in the window of interaction between the two rotors
Another feature of this invention is the ability to essentially eliminate any area of possible stagnation within the mixing chamber through the wing design of this invention and appropriate alignment of the rotors in the mixing machine. A still further feature of this invention is the ability to vary mixing intensities during the mixing cycle due to the geometries of the new winged rotors of this invention along with the rotor speeds employed.
The four wing rotor of this invention, for non-intermeshing synchronous rotation with an identical four wing non-intermeshing rotor in an internal, intensive batch mixing machine having synchronous drive means, comprises a rotor having an axis and an axial length from a first end of the rotor to an opposite second end of the rotor and having four wings of generally helical configuration including first and second long wings and first and second short wings. The first long wing originates at the first end of the rotor at about 0xc2x0 angular position relative to the rotor axis and has a wing tip oriented to the rotor axis at a helix angle of between about 45xc2x0 and 60xc2x0 and has an axial length of between about 60% to about 80% of the axial length of the rotor. The first long wing has an approach angle of from about 25xc2x0 to 60xc2x0. The second long wing on the rotor originates at the second end of the rotor at about 220xc2x0 to about 240xc2x0 angular position relative to the rotor axis and has a wing tip oriented to the rotor axis at a helix angle of between about 20xc2x0 to 40xc2x0 and has an axial length of between about 60% to 80% of the axial length of the rotor. The second long wing has an approach angle of between about 15xc2x0 to 25xc2x0. The wing tips of each of the first and second long wings have a width, as measured normal to the helix angle of the wing, with the width of the first long wing tip being at least 50% and up to about 100% wider than the width of the second long wing tip. The first short wing on the rotor originates at the first end of the rotor at an angular position in the range of about 170xc2x0 to about 190xc2x0 relative to the rotor axis, and has its wing tip oriented to the rotor axis at a helix angle of in the range of from about 25xc2x0 to 60xc2x0, and preferably essentially equal to the helix angle of the wing tip of the first long rotor, and has an axial length of between about 10% and 30% of the axial length of the rotor. The second short wing on the rotor originates at the second end of the rotor at an angular position in the range of about 350xc2x0 to about 20xc2x0 relative to the rotor axis, has its wing tip oriented to the rotor axis at a helix angle in the range of about 25xc2x0 to 60xc2x0, and preferably essentially equal to the helix angle of the wing tip of the second long rotor, and has an axial length of between about 10% and 30% of the axial length of the rotor. Each of the first and second long wings and first and second short wings have their wing tips an essentially equivalent radial distance from the axis of the rotor.
When two of these rotors are placed in a synchronously operated batch mixer, the rotors are oriented in the mixing chamber of the mixer such that the leading edge of the helical wing tip of the first long wing of the first rotor is located at a collar of the first rotor that is at an opposite end of the mixing chamber from the collar of the second rotor from where the leading edge of the helical wing tip of the first long wing of the second rotor is located. Additionally, the two rotors are positioned in the mixing chamber so that during their non-intermeshing counter rotation the leading edge of the helical wing tip of the first long wing of the second rotor trails the leading edge of the helical wing tip of the first long wing of the first rotor in rotating through the window of interaction between the two rotors by from 90xc2x0 to 180xc2x0, preferably about 90xc2x0. In this orientation of about 90xc2x0, and as a result of the twist angles of the wings of the two rotors, each rotor wing provides an effective wiping of the processing surfaces of the adjacent rotor thus providing effective material renewal on these surfaces. The large and small wings on the two rotors essentially completely wipe the entire region of the mixer in the space between the two rotors thus ensuring further enhancement of distributive mixing. Other angular positions of the aforementioned rotor alignments greater than 90xc2x0 and up to 180xc2x0 difference can be employed to promote other aspects of the mixing process, such as for example material uptake and discharge from the mixer. In such increased angular positions of the originating points of the leading edges of the helical wing tips of the first long wings on the two rotors, a wider open space between the two rotors is provided at one angular position followed by a complete sweep of the region by the other rotor wings when the latter cross the window of interaction of the two rotors.